1. Field of the Invention
The present invention is directed to a method and an apparatus for detecting premature atrial contraction (PAC), and in particular to such a method and apparatus employing a cardiac lead having a multi-electrode tip.
2. Description of the Prior Art
Analysis of signal morphology, and time offsets between recurring features of an IEGM signal, for the purpose of classifying the signal as representing a particular type of cardiac activity, are known from U.S. Pat. No. 6,308,095 and U.S. Pat. No. 6,266,554. In the known techniques described in these patents, either a single signal is analyzed as it is received over time, or multiple signals, respectively received from relatively widely spaced apart electrodes, are analyzed. Since the electrodes are spatially separated, the propagating wavefront arising due to cardiac electrical activity will completely pass a first of the electrodes in its propagation path before reaching subsequent electrodes in its propagation path. In the intervening propagation distance between the spaced apart electrodes, the signals may be corrupted by noise, and it may further be difficult to determine whether the xe2x80x9csamexe2x80x9d signal is being received at the subsequent electrode as was xe2x80x9cseenxe2x80x9d by the first electrode. This makes it difficult to analyze features of the respectively received signals relative to each other, because there is an uncertainty as to whether any differences in the respectively received signals which are identified are truly indicative of a particular type of cardiac activity, or instead arise due to changes in the signal as it moves along its propagation path.
This is particularly true with regard to a conventional lead with a unipolar configuration, wherein the cardiac lead has an electrode at its distal tip, and whereby the stimulator housing, or a portion thereof, is used as the indifferent or return electrode. Clearly the spacing between the distal tip of the cardiac lead and the stimulator housing is many times larger than the size of the propagating wavefront. Even in the case of a conventional bipolar configuration, wherein a single lead carries an electrode at its distal tip, and another electrode, such as a ring electrode, disposed slightly behind the distal tip electrode, the spacing between the tip electrode and the ring electrode will still be larger than the propagating wavefront.
An electrode lead for a cardiac pacemaker is disclosed in U.S. Pat. No. 5,306,292 which has a distal tip with a number of closely spaced electrodes thereon, with the remainder of the hemispherical surface of the distal tip of the electrode being non-conducting. Circuitry in the pacemaker housing, connected to the respective electrodes via the electrode lead cable, allows the total conductive area and geometry of the distal tip of the cardiac lead to be selectively varied, by activating the electrodes in different combinations. For example, the combination of electrodes (i.e. conductive surfaces) at the tip of the cardiac lead which provides the lowest stimulation threshold can be determined by an autocapture unit, so that energy consumption can be reduced.
Premature atrial contraction (PAC) is not an uncommon occurrence and, by itself, usually is not a cause for immediate concern, although its presence over an extended period of time may be an indication that the patient is a likely candidate for atrial tachyarrhythmias in the future.
A premature atrial beat generates a premature P-wave, which may or may not be conducted to the ventricles. The premature P-wave is often difficult to locate when it is superimposed on the preceding T-wave. If the premature P-wave is conducted to the ventricles, this usually occurs with a QRST configuration that is almost identical to the surrounding normal sinus beats. Some premature P-waves conducted to the ventricles, may closely simulate ventricular premature beats.
An IEGM which contains a P-wave originating from a premature atrial contraction which is not recognized as such, may be mis-diagnosed, which may in turn result in erroneous treatment.
It is therefore not only important to be able to reliably detect the presence of a PAC, but also to identify its source of origin, i.e., whether it arises from the left atrium or the right atrium.
It is an object of the present invention to provide a method and an apparatus which allow reliable identification of a premature atrial contraction.
It is a further object of the present invention to provide such a method and apparatus which additionally allow identification of the general location of the origination of a premature atrial contraction.
The above object is achieved in accordance with the principles of the present invention in a method and an apparatus employing a cardiac lead having a multi-electrode tip, wherein the electrodes are separated from each other but are spaced closely enough relative to each other so that respective unipolar electrical signals obtained from the different electrodes arise from the same propagating wavefront, and wherein these respective unipolar electrical signals are analyzed in terms of relative time offset and/or signal morphology in order to identify a premature atrial contraction.
As used herein, the term xe2x80x9cunipolarxe2x80x9d does not necessarily mean a signal referenced to the pacemaker housing, but means a signal referenced to a designated reference point, which is the same (i.e., is used in common) for all of the unipolar signals.
Because of their close spacing relative to each other, the individual electrodes (dots) on the distal tip of the cardiac lead will xe2x80x9cseexe2x80x9d the same wavefront, but that wavefront will arrive at different dots at slightly different times, so that the respective unipolar signals obtained from each electrode dot will be slightly shifted in time relative to each other. If the wavefront (i.e., the depolarization wave) arrives from the right, for example, the unipolar signal from the electrode dot at a right side of the distal tip will arrive at a detector connected to the lead in advance of the unipolar signal from the electrode dot at the left side of the distal tip. If the depolarization proceeds from the left, the opposite will occur. If the depolarization arrives in a direction substantially along the longitudinal axis of the distal tip (either from in front or behind the distal tip), all of the electrode dots will see the polarization at the same time, and there will be little, if any, time offset in the respective unipolar signals.
The respective unipolar signals which arise upon the occurrence of normal sinus activity (i.e., without any PAC), can be identified in advance, and deviations from this normal set of signals can then be determined and ascertained as representing an occurrence of a PAC. Moreover, such deviations from the normal signals can be classified as representing a PAC generated in the left atrium, as distinguished from a PAC generated in the right atrium.
The time differences (offsets) between the signals from respective pairs of electrode dots can be identified in several ways. In one embodiment the time offset, is found by identifying features of the different signals. The direction of the wavefront is calculated using simple trigonometric formulas, and the beat is classified based on this direction and on morphological information in the signals representing the beat.
In another embodiment, the components of the unipolar signals representing the depolarization in question are correlated with each other to identify different delays. This processing is undertaken retroactively, i.e., not in real time so that the signals can be correlated for a number of different delays. The more closely that two signals resemble each other, the higher their correlation. The highest correlation will be obtained for the delay representing the time it takes for the depolarization wavefront to proceed from one electrode to the next.